Router operating system
As with a computer system, the operating system of a router is an integral part of the device. In order to configure the router, it is important that we understand fully the operating system and its function. The operating system that we will be using is the Cisco Internetworking System (IOS for short)
This IOS is common to all Cisco devices, routers, switches, firewalls etc
You can also gain access to the routers IOS via the console cable, the AUX port (via a telephone line or via TELNET. However connections to a router via AUX or TELNET require that some configuration of the router must have first been carried out. Therefore initial configuration of a router must ALWAYS be done via the console cable
This is the router just having completed running the bootstrap program and has located an IOS file and is loading it.
The router should then by default look for a saved configuration file. (startup-config in NVRAM)If the router cannot find a configuration file, it enters system configuration dialog mode. This mode is a wizard type menu driven interface that you will encounter when you connect to a router and the startup-configuration cannot be found.
This could be that the router is new, or an administrator has issued a erase startup-config command followed by a reload (this clears the NVRAM)
When configuration dialog mode is finished, you can save a backup copy of the configuration file to NVRAM. This mode is rarely used by administrators, but it can be a quick way for a novice to get a router up and running.
If a startup-config configuration file has been saved in NVRAM then the router will not start the system configuration dialog but will load the config file into RAM and execute it one line at a time
The configuration commands start routing processes, supply addresses for interfaces and define other operating characteristics of the router.